The first (second and even third) colony on Mars would have to be very much an "indoor" one. The surface of Mars as it is at the moment is pretty inhospitable, with an atmospheric pressure of about 1% that of Earth's and no magnetosphere, meaning that anyone and anything that fancied breathing would have to be protected in a pressurised habitat. In order to truly make Mars habitable for humans, it is thought that terraforming will be the way forwards. Terraforming is a currently hypothetical process by which we would alter the atmosphere, temperature and ecology of another planet to create an Earth-like habitat. Seeing as Mars is thought to have once closely resembled conditions on Earth, we've got a good starting point. The terraforming of Mars would consist of three major phases: Building an atmosphere, retaining it and keeping it warm. In very basic terms, we have to initiate global warming. Mars' atmosphere is currently about 95% carbon dioxide and, as we all know, carbon dioxide is a greenhouse gas. There is also enough of the stuff locked up in the Martian ice caps as dry ice that, if we were able to release it into the atmosphere, it would raise the atmospheric pressure enough to eliminate the need for pressurised space suits. The release of the CO2 would also contribute to warming and it has been calculated that raising the temperature at the poles by just a few degrees would be enough to kickstart a runaway greenhouse effect. There are a number of ways we could increase the temperature on Mars. One suggestion is that we put mirrors in orbit around the planet to reflect more sunlight onto its surface, another is that we redirect small comets to crash into it. The impact itself would cause energy to be released as heat and, if we carefully selected the comets, could also deliver compounds such as ammonia which would further add to the greenhouse effect. Of course, if we fill the atmosphere with carbon dioxide, humans wouldn't be able to breathe it, but we could potentially use plants, algae and phytoplankton to convert it into oxygen, although this would rely heavily on maintaining a water supply. Further release of oxygen could be achieved by electrolysis, separating hydrogen and oxygen in water, or extracting it from the metal oxides that are abundant in Martian soil. If successful, the terraforming of Mars could transform it into a planet very similar to Earth. However, unless we found a way to simulate a magnetosphere, the effects would most likely be reversed as the solar winds blew the new atmosphere off into space, just like it did with the old one.
